AFDD Arc Fault Detection
BS 7671 Requirements Explained

An Arc Fault Detection Device (AFDD) is a protective device designed to detect dangerous electrical arc faults and disconnect the circuit before the arc can start a fire. Arc faults occur when electricity jumps across a gap between conductors — for example, through damaged cable insulation, at a loose terminal connection, at a cracked conductor, or through carbonised material in an aged accessory.

Arc faults are particularly dangerous because they can generate temperatures exceeding 6,000 degrees Celsius at the point of arcing — hot enough to ignite surrounding materials including cable insulation, timber, plasterboard, and soft furnishings. Yet the current flowing through an arc fault may be too small to trip a conventional MCB or fuse, and the fault does not produce the earth leakage that an RCD detects. This means a dangerous arc can persist for extended periods, gradually heating surrounding materials until a fire starts.

There are two types of arc fault that AFDDs protect against:

Electrical Safety First estimates that electrical faults cause around 14,000 house fires per year in the UK, with faulty wiring, cables, and connections being among the leading causes. AFDDs address a critical gap in protection that conventional devices cannot fill.

AFDDs use sophisticated electronic analysis to distinguish between dangerous arc faults and the normal arcing that occurs during everyday electrical operation. This distinction is critical — a light switch produces a brief arc every time it is operated, a motor produces arcing at its commutator brushes, and many electronic devices produce switching transients that look superficially like arcs. An AFDD must detect genuinely dangerous arcs without nuisance tripping on these normal events.

The detection method relies on analysing the high-frequency noise signature superimposed on the 50Hz power waveform. When an arc occurs, it produces a characteristic pattern of high-frequency current fluctuations (typically in the kilohertz to megahertz range) that differ from normal switching transients. The AFDD contains a microprocessor that continuously samples the current waveform and applies algorithmic analysis to identify these arc fault signatures.

The AFDD analyses several characteristics of the current waveform:

• High-frequency noise content — Arcs produce broadband high-frequency noise that normal loads do not. The AFDD monitors the frequency spectrum of the current for patterns consistent with arcing.
• Current irregularities at zero-crossing — Arcs tend to extinguish momentarily as the AC waveform passes through zero and re-ignite as the voltage rises, creating characteristic discontinuities in the current waveform.
• Duration and persistence — Normal switching events are brief (milliseconds). Dangerous arc faults persist for multiple cycles and show repeating patterns. The AFDD requires the signature to persist for a minimum number of half-cycles before tripping.
• Current amplitude changes — The AFDD monitors for the random amplitude variations characteristic of an unstable arc, as opposed to the stable, predictable current drawn by a normal load.

AFDDs are manufactured to BS EN 62606, which sets out the performance requirements and test methods. The standard defines specific arc fault test scenarios that the device must detect, and specific non-arc scenarios that must not cause tripping. This standardisation ensures consistent performance across manufacturers.

BS 7671 Regulation 421.1.6 was introduced in Amendment 2 to the 18th Edition and provides a recommendation (not a mandatory requirement) for the use of AFDDs. The regulation states that arc fault detection devices conforming to BS EN 62606 are recommended to provide additional protection against fire caused by arc faults in AC final circuits.

The regulation specifically recommends AFDDs for circuits in the following locations:

It is important to note that the regulation uses the word "recommended" rather than "shall" — AFDDs are not currently a mandatory requirement under BS 7671. However, the trend in the industry is clear: AFDDs are increasingly being specified by designers and clients, and it is likely that future amendments will strengthen the recommendation towards a requirement, as has happened previously with RCDs and SPDs.

Some local authority building control departments and fire officers are already requiring AFDDs for specific applications, particularly in HMOs and care homes. When working on these types of premises, check the specific requirements with the local authority before finalising the design.

An AFDD and an RCBO protect against completely different types of fault, and one is not a substitute for the other. In practice, most modern AFDD devices combine all three functions — arc fault detection, RCD, and MCB — into a single unit, but understanding the distinction is important.

The key point is that an AFDD addresses a protection gap that RCBOs cannot fill. A series arc fault produces current that is always less than the normal load current (so the MCB element will not trip) and creates no earth leakage (so the RCD element will not trip). Only the arc fault detection function can identify the characteristic signature of the arc and disconnect the circuit. For this reason, AFDDs are specified in addition to existing protection, not as a replacement.

When specifying AFDDs, most electricians choose combined AFDD/RCBO devices that replace the standard RCBO in the consumer unit. This provides all three levels of protection from a single device, occupying a single (wider) module position in the distribution board.

Installing AFDDs is straightforward from a wiring perspective — the device replaces a standard RCBO in the distribution board. However, there are several practical considerations that affect the design and specification.

• Board compatibility — AFDD/RCBO devices are wider than standard RCBOs (typically 2 modules instead of 1). The consumer unit or distribution board must have sufficient space. Some manufacturers produce boards specifically designed to accommodate AFDDs with the correct bus-bar configuration.
• Wiring method — Both the line and neutral conductors for the protected circuit must pass through the AFDD. This is similar to an RCBO installation — the neutral is connected to the device, not to the common neutral bar.
• One AFDD per circuit — Each final circuit that requires AFDD protection must have its own device. AFDDs cannot protect multiple circuits through a single unit because the arc detection algorithm needs to analyse the current signature of an individual circuit.
• Circuit length and cable type — Some AFDD manufacturers specify maximum circuit lengths or restrictions on cable types. Long circuits with high capacitance (such as lengthy runs of SWA cable) can affect the high-frequency analysis. Check manufacturer guidance for specific limitations.
• Testing — AFDDs have a built-in test button that simulates an arc fault to verify the trip mechanism. This should be tested at commissioning and during periodic inspection. There is currently no standardised instrument test for AFDDs — the built-in test is the primary verification method.

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The UK market for AFDDs has grown significantly since their introduction to BS 7671. The main manufacturers offering AFDD devices compatible with UK consumer units and distribution boards are:

The cost differential between a standard RCBO (typically £25 to £50) and an AFDD/RCBO (typically £80 to £150) is significant — roughly doubling or tripling the cost per circuit. For a typical 10-way domestic consumer unit, using AFDDs on every circuit would add approximately £500 to £1,000 to the installation cost. This is the main barrier to wider adoption and the reason the regulation currently recommends rather than requires AFDDs.

Many electricians take a pragmatic approach, installing AFDDs on the highest-risk circuits — bedrooms, living rooms, and circuits running through combustible building elements — while using standard RCBOs on lower-risk circuits such as the cooker and immersion heater.

Before specifying AFDDs on a job, there are several practical factors to consider beyond the regulatory recommendation.

As the technology matures and costs reduce — a trend that has already begun — AFDDs will likely become as standard as RCDs are today. Forward-thinking electricians are already gaining experience with AFDDs and discussing them with clients as part of the design process. The RCD types guide covers the complementary protection provided by different RCD types alongside AFDDs.

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